Interstory Drift Calculator

Calculator Inputs

Project Name

Load Case

Displacement Unit

Calculation Mode

Warning Drift Ratio

Limit Drift Ratio

Amplification Factor

P-Delta Factor

Apply P-Delta Adjustment

Notes

Single Story Inputs

Story Name

Story Height

Lower Floor Displacement

Upper Floor Displacement

Formula Used

Elastic Drift = | Upper Floor Displacement − Lower Floor Displacement |

Adjusted Drift = Elastic Drift × Amplification Factor × P-Delta Factor

Interstory Drift Ratio = Adjusted Drift ÷ Story Height

The calculator first finds the relative lateral displacement between two adjacent floors. It then applies any selected amplification and second-order adjustment factors. Finally, it divides the adjusted drift by the story height to compute the interstory drift ratio.

You can compare the result against your selected warning and limit thresholds to flag serviceability or code-check concerns during structural review.

How to Use This Calculator

Enter the project name, load case, and displacement unit.
Choose either single story mode or bulk multi-story mode.
Input story height and floor displacements for each evaluated level.
Set warning and limit ratios to match your design criteria.
Apply amplification and P-Delta factors when required by your workflow.
Click calculate to see summary metrics, detailed results, and the chart.
Use CSV export for spreadsheets and PDF export for reporting.
Review the critical story and compare ratios against permitted limits.

Example Data Table

Story	Height	Lower Disp	Upper Disp	Elastic Drift	Drift Ratio
Story 1	3200	0.0	8.2	8.2	0.002563
Story 2	3200	8.2	17.5	9.3	0.002906
Story 3	3200	17.5	28.1	10.6	0.003313
Story 4	3200	28.1	41.3	13.2	0.004125

This example assumes unity amplification and no additional second-order factor. Modify limits and factors to reflect your design standard.

Frequently Asked Questions

1. What is interstory drift?

Interstory drift is the relative lateral displacement between two consecutive floors. Engineers use it to assess deformation, occupant comfort, partition damage risk, and overall structural performance.

2. Why is drift ratio more useful than raw drift?

Raw drift depends on story height, so comparison across floors can mislead. Drift ratio normalizes deformation by height, making performance checks more consistent and meaningful.

3. When should I use an amplification factor?

Use an amplification factor when your analysis procedure or design standard requires converting elastic analysis results into expected design-level displacements for drift evaluation.

4. What does the P-Delta factor represent?

The P-Delta factor approximates second-order effects from gravity loads acting through lateral displacements. It can increase drift demand in taller or more flexible structures.

5. Can I check an entire building at once?

Yes. Use bulk mode and enter one story per line with story name, height, lower displacement, and upper displacement. The calculator processes every row together.

6. What drift limit should I use?

That depends on the governing code, structural system, nonstructural elements, and load case. This tool lets you enter your own warning and failure thresholds.

7. Does this calculator replace full structural analysis software?

No. It is a checking and reporting aid. Final design decisions should still come from validated structural analysis models, code provisions, and professional engineering judgment.

8. What exports are available?

You can export the calculated table as CSV for spreadsheet work and generate a PDF report from the browser for review, documentation, or project submission.